Re-configurable SLMs based on liquid crystal (and other types of) devices are widely used for controlling and manipulating optical beams. In diffractive mode they may be used for three dimensional (3D) imaging [BROWN, C V and STANLEY, M, UK Patent Application GB2330471, Production of Moving Images for Holography] and for routing optical signals in telecommunications networks [See for example ROSES (Re-configurable Optical Switches) website, http://www-g.eng.cam.ac.uk/photonics/rose1.html].
The SLM.modulates the complex amplitude of an incoming wave front (i.e. changes its phase and/or amplitude), which causes it to propagate in the desired manner. The SLM generally comprises a liquid crystal panel containing a number of individually addressed pixels, onto which a diffraction pattern or Computer Generated Hologram (CGH) is written [CAMERON, C D et al, SPIE Conference on Critical Technologies for the Future of Computing (San Diego, USA), July-August 2000, Computational Challenges of Emerging Novel True 3D Holographic Displays]. 
CGH 3D display systems typically use a computer to generate and/or store electronic copies of the hologram. This hologram is then replayed on an SLM which is switched to modulate (in transmission or reflection) light from a source which then passes through suitable replay optics, thereby providing a visible three-dimensional image to observers.
For many image generation applications, especially holographic 3D image generation, it is important to maximise the image size and/or the range of angles over which the image can be viewed. Conventionally, this is achieved by increasing the spatial frequency content of the hologram, which increases the achievable diffraction angle of the modulated beam and/or increasing the number of pixels in the SLM. In order to produce satisfactory images, SLMs containing of the order of 1010 pixels may be required. An increased spatial frequency is also desirable for many other applications utilising diffractive SLMs, such as optical switching.
HAINES and BRUM [Proceedings of the IEEE (Letters), p 1512-3, August 1967, A Technique for Bandwidth Reduction in Holographic Systems] proposed using a ground glass scatter plate as a means of enhancing the viewing angle without having to increase the spatial frequency content. The principle was successfully demonstrated using a conventional fixed, photographically recorded hologram of a point source hologram (note: the scatter plate is used in both the recording and re-construction stage). KOMAR [SPIE Vol 120, p 127, 1977, Progress on the Holographic Movie Process in the USSR] discussed the use of the scatter-plate technique as a means of producing holographic “movies” in which the viewing angle was big enough for a large audience to see the 3D images. He concluded that the technique caused unacceptable degradation in the image quality.
The main factor preventing the introduction of reconfigurable computer generated holograms (CGH) in many applications is the number of addressable, reconfigurable pixels required in such devices. This is particularly important in the area of 3D image generation.
To ensure adequate fields of view (FOV) and image sizes, conventional approaches typically need a CGH having a pixel count several orders of magnitude higher than that required to produce image resolutions that the human visual system can perceive. The simple relationFOV.I˜n λ/4  (1)(where n is the number of pixels across the display in the plane where the FOV is specified, I is image width, and λ is the wavelength of light generating the real image) shows that, for typical applications (e.g. FOV=±30°, I=0.05 m), ˜1010 pixels are required to be addressed. This number is enormous. Any method allowing a pixel count reduction, without significantly compromising perceived image qualities, will have great effect on the practicality of such systems.Statement of Invention.
The above problem of a very large pixel count is reduced, according to the present invention, by the use of a scatter plate of known feature composition together with a computer generated diffractive pattern or hologram calculated to pre-compensate for effect of the scatter plate.
According to this invention, a reconfigurable spatial light modulator system comprises:    a controller for holding a compensated pattern;    a first spatial light modulator having a plurality of addressable pixels controlled by the controller each pixel being capable of modulating incident light and collectively replicating the compensated pattern;    a scatter plate of known characteristics for scattering light from the first spatial light modulator;    optical means for directing light scattered by the scatter plate and presenting a pattern to a receiver;    the compensated pattern being related to both the scatter plate characteristics and to the pattern presented to the receiver.
The scatter plate may be a simple binary phase pseudo random diffuser and may have a number of pixels much greater than the number of pixels on the first SLM. Other formats may be used. For example multi level pseudo random devices and diffusers with periodic structures.
The first spatial light modulator may be an electrically addressable liquid crystal spatial light modulator (EASLM) operable either in transmissive or reflective mode.
The optical means may be a lens system, e.g. a Fourier Transforming lens.
The system may include a second SLM which receives modulated light from the first SLM; the scatter plate 5 is associated with the second SLM in such systems. This second SLM may be optically addressable (OASLM) having substantially the same or different number of addressable pixels as the first SLM, and may be of greater dimensions to give a magnified image or smaller dimensions to provide increased spatial frequency. Alternatively, the second SLM may be formed by a number of individual OASLMs arranged in a x,y matrix or tiled configuration. When the second SLM has a tiled configuration, then scanning means are included for directing scattered light to different tiled areas in turn.
The controller may be a computer having memory for storing basic images and calculating means for providing compensated pattern from basic images to take account of scattering by the scatter plate. The basic images may be obtained by scanning of actual objects, in a manner analogous to forming holographic images in film material, conventional holography as in U.S. Pat. No. 6,078,392, direct to digital holography, or by calculation as in computer aided design (CAD) processes. Alternatively the computer may have a memory unit for storing images previously processed elsewhere, and then read out into the SLM as required.
The system may display holographic images directly to one or more observers. Alternatively, the system may further include a receiver, which may be an array of detector elements, a bundle of optical fibres, a screen onto which an image is projected for observation by one or more observers. The light source may be ambient light, or light from a single or multiple light sources such as one or more optical fibres or lasers.
According to this invention a method of providing a holographic image to an observer includes the steps of:    providing a holographic engine for storing a computer generated hologram pattern of an image to be displayed    providing a spatial light modulator having a large number of addressable pixels each capable modulating light under the control of the engine    controlling the spatial light modulator so that the observer receives a holographic image;Characterised By    providing a light scattering plate of known characteristics to modify light from the spatial light modulator;    calculating and providing a compensated computer generated hologram pattern of an image to compensate for the known characteristics of the scattering plate so that an observer receives a holographic image.
The compensated CGH pattern may be calculated using direct binary search algorithm.
According to another aspect of this invention a method of increasing the range of diffraction angles from a computer designed diffraction structure includes the steps of:    providing a holographic engine for storing a computer generated pattern of a structure to be displayed;    providing a spatial light modulator having a large number of addressable pixels each capable of modulating light under the control of the engine;Characterised By    providing a light scattering plate of known characteristics to modify light from the spatial light modulator;    calculating and providing a compensated computer generated hologram pattern of a diffractive structure to compensate for the known characteristics of the scattering plate so that an increased range of diffraction angles are obtained.